In radiography the interior of objects is reproduced by means of penetrating radiation which is high energy radiation belonging to the class of X-rays, γ-rays and high energy elementary particle radiation, e.g. β-rays, electron beam or neutron radiation. For the conversion of penetrating radiation into visible light and/or ultraviolet radiation luminescent substances are used called phosphors.
In a conventional radiographic system an X-ray radiograph is obtained by X-rays transmitted imagewise through an object and converted into light of corresponding intensity in a so-called intensifying screen (X-ray conversion screen) wherein phosphor particles absorb the transmitted X-rays and convert them into visible light and/or ultraviolet radiation whereto a photographic film is more sensitive than to the direct impact of the X-rays. In practice the light emitted imagewise by said screen, whether or not provided with a reflecting layer and/or a fluorescent dye layer in favor of speed or lowering of phosphor coating amount as e.g. in EP-A 0 595 089, irradiates a contacting photographic silver halide emulsion layer film which after exposure is developed to form therein a silver image in conformity with the X-ray image.
As another development described e.g. in U.S. Pat. No. 3,859,527 an X-ray recording system is disclosed wherein photostimulable storage phosphors are used that, in addition to their immediate light emission (prompt emission) on X-ray irradiation, have the property to temporarily store a large part of the energy of the X-ray image which energy is set free by photostimulation in the form of light different in wavelength characteristic from the light used in the photostimulation. In said X-ray recording system the light emitted on photostimulation is detected photo-electronically and transformed in sequential electrical signals.
The basic constituents of such X-ray imaging system operating with storage phosphors are an imaging sensor containing said phosphor, normally a plate or panel, which temporarily stores the X-ray energy pattern, a scanning laser beam for photostimulation, a photo-electronic light detector providing analog signals that are converted subsequently into digital time-series signals, normally a digital image processor which manipulates the image digitally, a signal recorder, e.g. magnetic disk or tape, and an image recorder for modulated light-exposure of a photographic film or an electronic signal display unit, e.g. cathode ray tube. A survey of lasers useful in the read-out of photostimulable latent fluorescent images has been given in the Research Disclosure Volume 308 No. 117 p.991, 1989.
From the preceding description of said two X-ray recording systems operating with X-ray conversion phosphor screens in the form of a plate or panel it is clear that said plates or panels only serve as intermediate imaging elements and do not form the final record. The final image is made or reproduced on a separate recording medium or display. The phosphor plates or sheets provide ability to repeated re-use. Before re-use of the photostimulable phosphor panels or sheets a residual energy pattern is erased by flooding with light.
From the point of view of image quality of the image storage panels, especially with respect to sharpness, the said sharpness does not depend upon the degree of spread of the light emitted by the stimulable phosphor in the panel, but depends on the degree of spread of the stimulable rays in the panel: in order to reduce this spread of light a mixture can be made of coarser and finer batches to fill the gaps between the coated coarser phosphor particles. A better bulk factor may be attained by making a mixture of coarser and finer phosphor grains resulting in a loss in sensitivity unless the said phosphor grains are only slightly different in sensitivity. For intensifying screens this topic has already be treated much earlier by Kali-Chemie and has been patented in U.S. Pat. Nos. 2,129,295; 2,129,296 and 2,144,040. Radiographs showing improved visualisation, comprising therefore a blue-light absorbing (yellow) dye have been described in EP-A 0 028 521.
Especially the phosphor layer thickness can give rise to increased unsharpness of the emitted light, this being the more unfavorable if the weight ratio between the amount of phosphor particles and the amount of binder decreases for the same coating amount of said phosphor particles.
Measures in order to provide sharper images by enhancing the weight ratio amount of phosphor to binder, e.g. by decreasing the amount of binder, however lead to unacceptable manipulation characteristics of the screen, due to insufficient elasticity and brittleness of the coated phosphor layer in the screen.
One way to get thinner coated phosphor layers without changing the coated amounts of pigment and of binder makes use of a method of compressing the coated layer containing both ingredients at a temperature not lower than the softening point or melting point of the thermoplastic elastomer as has been described in EP-A 0 393 662. Another way free from compression manufacturing techniques has been proposed in WO 94/0531, wherein the binding medium comprises one or more rubbery and/or elastomeric polymers providing improved elasticity of the screen, high protection against mechanical damage, high ease of manipulation, high pigment to binder ratio and an improved image quality, especially sharpness. Early references referring to the improvement of sharpness of radiation image storage panels are related with the addition of a colorant to the panels. So in U.S. Pat. No. 4,394,581 a dye or colorant is added to the panel so that the mean reflectance of said panel in the wavelength region of the stimulating rays for said stimulating phosphor is lower than the mean reflectance of said panel in the wavelength region of the light emitted by said stimulable phosphor upon stimulation thereof.
In U.S. Pat. No. 4,491,736 more specifically an organic colorant is disclosed which does not exhibit light emission of longer wavelength than that of the stimulating rays when exposed thereto. So EP-A 0 165 340 and the corresponding U.S. Pat. No. 4,675,271 disclose a storage phosphor screen showing a better image definition by incorporation of a dye. An analoguous effect brought about in phosphor layers of image storage panels by incorporation of dyes or colorants has further been described in EP-A 0 253 348 and the corresponding U.S. Pat. No. 4,879,202 and in EP-A 0 288 038.
In order to further improve image resolution a radiation image storage panel colored with a dye has been disclosed in EP-A 0 866 469 and the corresponding U.S. Pat. No. 5,905,014 having as a characteristic feature that as a colorant a triarylmethane dye having at least one aqueous alkaline soluble group is present in at least one of said the support, the phosphor layer or an intermediate layer between said support and said phosphor layer. As presence of the colorant in the storage phosphor layer may lay burden on screen speed (sensitivity), it is preferred to apply the dye in the intermediate layer between support and phosphor layer and/or in the support only. Presence in the intermediate layer between support and phosphor layer, also called “antihalation undercoat” or “AHU” wherein said support is a reflective support, clearly offers the best relations with respect to speed and sharpness. A disadvantage however resulting from the (mostly obligate) use of differing binder material in both of said storage phosphor and AHU layers is its lack for a perfect adhesion of both layers, more especially in view of manipulation and repeated use (re-use) of storage phosphor panels.